This invention relates to surfactant flooding fluids for the enhanced recovery of petroleum from porous subterranean reservoirs.
The petroleum industry has long recognized that only a fraction of the original oil in a reservoir is expelled by natural mechanisms or primary production. Accordingly, there is a need for improved methods of increasing the ultimate yield of petroleum from natural reservoirs. Many large reserves of petroleum fluids from which only small recoveries have been realized by present commercial recovery methods are yet to reach a potential recovery approaching their estimated oil-in-place.
The production of petroleum products is usually accomplished by drilling into a hydrocarbon-bearing formation and utilizing one of the well-known recovery methods for the recovery of hydrocarbons. However, it is recognized that these primary recovery techniques may recover only a minor portion of the petroleum products present in the formation particularly when applied to reservoirs of viscous crudes. Even the use of improved recovery practices involving heating, water flooding and steam processing may still leave up to 70-80 percent of the original hydrocarbons in place.
Water flooding is one of the more widely practiced secondary recovery methods. A successful water flood may result in recovery of 30-50 percent of the original hydrocarbons left in place. However, generally the application of water flooding to many crudes results in much lower recoveries.
The application of these secondary recovery techniques to depleted formations may leave major quantities of oil-in-place, since the crude is tightly bound to the sand particles of the formation, that is, the sorptive capacity of the sand for the crude is great. In addition, interfacial tension between the immiscible phases results in entrapping crude in the pores, thereby reducing recovery. Therefore, various methods of tertiary or enhanced recovery have been proposed.
Various additives can be added to the flood water to increase oil displacement. For instance a surfactant such as a petroleum sulfonate may be added to the water to lower the interfacial tension between oil and water. If enough surface active ingredient is added to lower the interfacial tension sufficiently, then more oil can be displaced from a reservoir by the water containing the surfactant than can be displaced by water not containing a surface active agent. If addition of a surface active agent lowers the interfacial tension to a value of 0.01 dynes per centimeter or lower, then water containing the surface active agent will displace essentially all of the oil from most of the reservoir. In contrast water not containing a surfactant will leave an oil saturation in the reservoir usually between 15 and 50 percent of the pore volume. Thus, the purpose of adding a surfactant such as a petroleum sulfonate to water is to increase the microscopic displacement of oil from the volume of the reservoir rock contacted by the water.
Another additive commonly employed in such fluids is a polymer. The purpose of adding polymer to the surfactant fluid is to decrease the mobility of the fluid in the reservoir. This increases the volumetric sweep of the surfactant solution or, in other words, allows the solution to contact a much larger volume of the reservoir than it would without the polymer. The polymer also enhances the formation of an oil bank in front of the surfactant slug. Thus both surfactant and polymer are usually considered required to ensure both good macroscopic and microscopic (or volumetric) displacement of oil.
U.S. Pat. No. 4,049,054 discloses a method of preparing a stable surfactant water flooding fluid comprising contacting a polymer with fresh water in the substantial absence of a salt and thereafter combining the thus formed polymer solution and a salt solution and finally adding a surfactant. It is thought the particular steps of mixing the components provides a more stable mixture for injection.
U.S. Pat. No. 4,252,19 discloses a process for enhanced oil recovery using the product obtained by mixing a major proportion of a petroleum oil feed stock, and a minor proportion of an additive, such as an oxygenated hydrocarbon, with SO.sub.3 under sulfonation conditions, mixed with about 0.5 to 20% of water at about 50.degree. to 150.degree. C. for a relatively brief period of time and then neutralizing the resultant material with a base.
In many of the enhanced oil recovery methods known in the art, where a surfactant is employed in the flood operation, a generally accepted practice is to follow the surfactant solution with a polymer. Contrary to the intended result, often the polymer does not follow the same path as the surfactant, tends to "finger" and bypass the surfactant and therefore much surfactant and potentially recoverable oil is left behind when the polymer is produced. Several reasons for this are proposed. The necessity of the surfactant in the first place is due to the immiscibility of water used in waterflooding with the oil which is supposed to be recovered. However, it is common to add a surfactant to lower water/oil interfacial tension and then follow the fluid with a polymer that is not miscible with that surfactant fluid.
To make surfactant flooding economically viable, surfactants must be inexpensive. Commercial sulfonates such as those produced by WITCO.RTM. and STEPAN.RTM. are most commonly used because of their price. Unfortunately, these products are mixtures of oil and water which contain soluble poly-sulfonates as well as mono-sulfonates. It has been shown in the literature that sulfonate mixtures containing high concentrations of mono-sulfonate have lower interfacial tensions with oil than mixtures having higher concentrations of poly-sulfonates. By using the process of this invention wherein a polymer is added to the brine-sulfonate-solubilizer fluid, the mixture allowed to separate, and the fraction with the highest sulfonate content used in the flooding operations, the surfactant fluid, has a higher concentration of mono-sulfonates, resulting in a lower interfacial tension with the reservoir oil, therefore the tertiary recovery procedure is more efficient.
In the invention disclosed herein, it is believed that the efficiency of flood operations employing a surfactant can be improved, because of the lower interfacial tension between the fluids used. Some of the disadvantages of the prior art are avoided and there is a greater percentage of oil produced by enhanced or tertiary recovery.